1. Field of the Invention
The present invention relates to a semiconductor device.
Note that a semiconductor device in this specification refers to general devices which can function by utilizing semiconductor characteristics; for example, a semiconductor integrated circuit including a semiconductor element such as a transistor, an electro-optical device such as a display device, and an electronic device are all semiconductor devices.
2. Description of the Related Art
The scale of semiconductor integrated circuits including logic circuits has increased year by year. Integrated circuits were constituted by several elements at an early stage of development; nowadays, there are central processing units (CPUs) and digital signal processors (DSPs) constituted by tens of millions of elements.
Although power consumption per element is reduced by miniaturization and reduction in driving voltage of elements included in such CPUs and DSPs, power consumption of the entire integrated circuit is being increased because the number of elements is more increased.
As a method for reducing power consumption of the integrated circuit, clock gating by which clocks are partly stopped, a method for lowering the clock frequency, and a method for lowering power supply voltage partly have been developed.
Power consumption includes static power due to leakage current of an element such as a transistor in the off state, as well as dynamic power due to charge and discharge caused by switching of the element. The static power can be almost negligible in integrated circuits with a small number of elements, but is too high to ignore in integrated circuits with an enormous number of elements. Thus, for an integrated circuit with high static power, the following method (power gating) is developed: supply of a power supply voltage to circuits in the integrated circuit is controlled by switches (also referred to as power supply control switches or power gates). By this method, static power due to leakage current of an element such as a transistor can be reduced, and moreover, power consumption of the integrated circuit can be reduced.
For example, Patent Document 1 discloses a semiconductor integrated circuit that can implement power gating. Specifically, Patent Document 1 discloses a semiconductor integrated circuit that includes a transistor between a logic circuit and a power supply line, and can control supply of a power supply voltage to the logic circuit by controlling switching of the transistor.
In addition, Patent Document 2 discloses a microcomputer or the like which can stop supply of a power supply voltage to a CPU, a memory, and the like in a period during which supplying power is not necessary, for example.